The relation between the atomic mass ratio and quartic anharmonicity in alkali metal hydrides

Phonons play a crucial role in understanding various aspects of solid-state physics, including thermal expansion, phase transition, interfacial thermal resistance, and lattice thermal conductivity (κ_L). Usually, the three-phonon (3ph) scattering processes have been considered the dominant mechanism governing thermal transport in solids. However, recent studies have revealed that quartic anharmonicity can fill the gap between theoretical calculations based on 3ph interactions and experimental values in a range of materials, such as BAs, perovskite CsPbBr₃, and thermoelectric PbTe. Typically, the vibration frequency of phonons is proportional to the inverse square root of the atomic mass. Some semiconductors with heavy atoms have a big mass ratio which leads to a gap between acoustic and optical phonon modes. This phenomenon concurrently suppresses 3ph scattering. The influence of 4ph scattering on κ_L becomes more important. We investigate the relation between the atomic mass ratio and quartic anharmonicity using rocksalt alkali metal hydrides XH (X = Li, Na, K, Rb, Cs). Our finding reveals a positive correlation between the atomic mass ratio and quartic anharmonicity.